This disclosure relates to a capacitor discharge circuit for discharging charges accumulated in a capacitor connected between alternating-current power lines.
Power supply circuits for electric devices of related art, a capacitor called an across-the-line-capacitor (hereinafter abbreviated as “X capacitor”), which suppresses unnecessary radiation, is provided between the opposite ends of an alternating-current power source. A capacitor discharge circuit described in Japanese Patent No. 4446136 has been known as a conventional capacitor discharge circuit including an X capacitor.
This capacitor discharge circuit includes: an X capacitor connected between alternating-current power lines; and a series circuit including a first capacitor and a second capacitor that are connected between the alternating-current power lines and detect the voltage of an alternating-current power source. To this series circuit are connected a first time-constant circuit including a third capacitor, a first resistor, and a second resistor, and a second time-constant circuit including a fourth capacitor, a third resistor, and a fourth resistor. The time constant of the first time-constant circuit is set to be less than the time constant of the second time-constant circuit. Accordingly, the charges of the third capacitor are discharged more quickly than the charges of the fourth capacitor.
A transistor is connected to the outputs of the first time-constant circuit and the second time-constant circuit. A series circuit including a discharge resistor, a first switch, and a second switch is connected between the opposite ends of the X capacitor. The collector of the transistor is connected to the gates of the first switch and the second switch, and the emitter of the transistor is connected to the node between the first switch and the second switch.
Here, when the plug is pulled out from a commercial power source, the charges of the third capacitor are discharged through the first resistor and the second resistor and the transistor is turned off, but the charges of the fourth capacitor remain. Then, by the charges remaining in the fourth capacitor, the voltage of fourth capacitor is applied to the gates of the first switch and the second switch.
Thus, the first switch and the second switch are turned on, and the charges of the X capacitor are discharged through the discharge resistor. As a result, there is no potential difference between the input terminals of the power supply circuit. Hence, the user can be prevented from getting an electric shock by touching the plug.
Meanwhile, alternating-current voltage detection is performed to discharge the charges remaining in the X capacitor when the commercial power source is turned off. The alternating-current voltage detection is generally performed by dividing the voltage of the commercial power source by using resistors.